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flues open before it, entirely forgetful of the fact that in gases, by
reason of their high coefficient of expansion due to temperature
changes, great variations of density occur. At a temperature of
1638° C. a cubic meter of air weighs 0.171 kg, while at 0° C. the
same volume of air will weigh 1.293 kg or 7.5 times as much.
The relative densities may be compared by likening the high-
temperature air to water, weighing 62.5 Ib per cubic foot, while
3 Hot Gases Rising:
[^[Gases Chilled by Contact
•iiiil with water cooled
jjJEddy Areas of Partially Surfaces fallins
FIG. 168.—Diagram of Gas Flow in Stirling Type Boiler.
the low-temperature air may be likened to molten iron, which
weighs 430 Ib per cubic foot or nearly 7 times the weight of water.
One thing that has tended to obscure this great difference in
density that occurs in gases is the fact that most engineering
works, in considering the gases passing through a boiler, supply
the information as weight of gases, allowing the investigator to lose
sight of the fact that a unit weight of gas in the firebox will
occupy approximately three times the volume it does when it